[Coco] Re: Floppy contoler Q

[Dave P.]

On Sun, 2006-01-15 at 14:20 -0500, Arthur Flexser wrote:
> Actually, I do recall hearing of someone who hooked in some logic to the select
> lines to enable (I think) 4 double-sided drives, with the extra one being
> enabled whenever both DS1 and DS2 were active ($06 and $46 for its 2 sides).
> A practical reason to do this, back in the olden days, would be if you were
> running a BBS from floppies, which would require a goodly amount of storage.

Since I know of nobody else who did that hack, I assume you're taking
about me. I did modify my Super Controller II to handle 4 double sided
drives, but I used DS0 and DS1 to generate the real DS3 signal. I have
pictures and schematics of my modifications at
<http://wormfood.net/old_computers/coco/> if anyone wants to see what
exactly I did. I know others have been interested in this, and I was
working on a web page to explain all the details, but never got around
to finishing it.

I went searching for floppy connector pinouts a while ago, and it seems
there are are least 2 different standards for them. The one Tandy choose
to use is the less popular standard, that uses their DS3  on the same
pin everyone else uses for SS (side select). A fringe benefit of this
is, while it was designed for 4 single sided drives, it can control 3
double sided drives with just software changes.

What happens if you connect 4 double sided drives to a stock floppy
controller?
You get access to 3 of the drives, since the 4th drive will have no DS
signal to select it.

What happens if you connect 3 double sided drives, and a 4th single
sided drive to a stock controller?
All drives work normally as single sided drives, but as soon as you try
to select the 2nd side of one of the double sided drives, you will also
select the 4th drive, and both floppy drives will try to drive the
floppy bus at the same time. (while that may sound bad, it really won't
hurt the hardware, but the software won't like it.)

What if someone seriously wanted to run more than 4 floppy drives?
I would expect it to require a lot more work to the controller than just
the additional DS signals. The floppy controller was only designed to
drive a bus with a max of 4 floppy drives on it. Even if you modified
the driver/buffer circuit in the controller to have more power, you
still have to deal with the floppy drives, which also drive the same
bus, and are unlikely to have been designed to drive a bus with more
than 4 other devices on it.

-Dave